SF has 121km^2 of land area within the city proper. This may be a little low - I was hoping to use the area-including-water of 601km^2 within the city limits.

SF has 121km^2 of land area within the city proper. This may be a little low - I was hoping to use the area-including-water of 601km^2 within the city limits. Kowloon Walled City was put together with little central planning, ruled by gangs, located directly under the approach path of an airport landing strip, and relied on a near-zero-public-infrastructure model. It was limited to ~15 stories. It housed ~50k people on 6.5 acres. Most importantly, it was built for people in the developing world, for a lot less than $2M per dwelling - residents accepted about $10k per person in remuneration during the eviction/demolition process; If we take this as a fair value of the housing construction, we have perhaps 50x the resources as Kowloon had. That density at 121km^2 is 230 million people in the city landmass of SF; At 601km^3 it rises to 1.14 billion. For 50x the resources, in a streamlined design-build process rather than an ad-hoc "does the roof break? No? Build higher" manner, with an indefinite height limit and modern technology and a large degree of central planning (absolutely necessary for a 3D construction+utilities grid), I assert we could comfortably house the vast majority of the world in 601km^2 if we absolutely had to at 6x the density of Kowloon. 50x the resources pays for a lot of complexity. Right now, that money (an enormous quantity of the urban economy) is draining into land values & the financial system, but the city is enduring that money sink because the city (and large cities, in general) has found things it is fantastically more productive at, than 100 large towns each 1% of the size.